1. Field of the Invention
This invention relates to instrument systems for use with industrial processes. More particularly, this invention relates to such systems wherein a measurement signal responsive to a process condition, such as differential pressure or the like, is transmitted from a field location to a signal-receiving location such as an instrument control room. The invention is illustrated in the preferred embodiment described hereinbelow for use in transmitting signals developed by a force sensor of the vibrating-wire type.
2. Description of the Prior Art
Force sensors of the vibrating-wire type have been known in the art for many years. Recent developments in such sensors have resulted in greatly improved performance capabilities, so that the use of vibrating-wire instrumentation has begun to increase significantly in the process industries. One particularly advantageous instrument of that type is disclosed in copending U.S. application Ser. No. 834,481, now U.S. Pat. No. 4,165,651 filed by E. O. Olsen et al on Sept. 19, 1977.
To use a vibrating-wire sensor as part of a modern instrument system requires that an electric-signal transmitter be provided for the sensor to develop a suitable signal corresponding to the force applied to the vibrating wire. One such transmitter is disclosed in U.S. Pat. No. 4,118,977, issued to E. O. Olsen et al on Oct. 10, 1978. In that transmitter, electronic circuitry for the vibrating-wire sensor includes an oscillator coupled through a two-wire line to the vibrating wire for inducing vibrations at the wire resonant frequency, and frequency-to-analog converter means for converting the oscillator frequency to a corresponding direct-current signal, e.g. in the range of 4 to 20 ma, suitable for transmission to a central station by a second two-wire line. The electronic circuitry in that arrangement ordinarily is located immediately adjacent the sensor, but may be remote from the sensor by a moderate distance which is limited by the characteristics of the two-wire line coupled to the vibrating wire.
The transmitter circuitry shown in U.S. Pat. No. 4,118,977 also includes additional signal-processing means for scaling the d-c signal to correspond with a prescribed measurement range, and for characterizing the signal to provide a linear relationship with respect to the applied force. Thus, the final d-c measurement signal can be utilized compatibly with the apparatus of established instrument systems employing similar direct-current sensor signals, control signals, and the like, for example computer-operated systems such as disclosed in copending U.S. application Ser. No. 737,195, now abandoned filed by R. A. Williamson, Jr., and R. W. Ford on Oct. 29. 1976.
Although known transmitter apparatus as described above has performed well, improvements are desirable in certain important respects. For example, it has been found to be particularly advantageous to reduce the amount of electronics required at the transmitter location. It also has been found quite desirable to be able to transmit additional information on the same transmission channel between the field station and the central station. Additionally, experience has shown the need to provide at the transmitter a simple and inexpensive means for indicating to field service personnel the actual value of the measurement as it appears at the distant control room. The present invention is directed to providing improvements in these and other respects.